A work machine such as, for example, an off-highway truck, a loader, a motor grader, or any other work machine known in the art may include an engine connected to a multi-speed bidirectional transmission that automatically shifts gears according to predetermined shift maps or that is manually shifted between gears by a work machine operator. These gear shifts may be initiated regardless of the magnitude of a rotational speed or direction differential between the engine and the transmission or regardless of the magnitude of a travel speed of the work machine. Gear shifts according to this method are inefficient and may result in excessive wear to driveline components and undesirable vibration experienced by the work machine operator.
One method to reduce vibration and the resulting wear is to bring the engine speed within a predetermined amount of the transmission speed prior to full transmission gear engagement. One such transmission control system is described in U.S. Pat. No. 5,583,766 (the '766 patent) issued to Birchenough et al. on Dec. 10, 1996. The '766 patent teaches a control system for a vehicle that regulates engine speed to a predetermined amount above a synchronization speed (equal engine and transmission speeds) during an upshift and to a predetermined amount below the synchronization speed during a downshift. The amount above and below the synchronization speed may be determined through testing to minimize stress on the driveline components without resulting in unacceptable shift hesitation and torque interruption, and may be dependent on the performance characteristics of the engine, torque converter, and transmission.
Although the vehicle control system of the '766 patent may reduce some of the vibration and wear associated with transmission gear shifting, the vehicle control system also reduces engine speed during downshifting to a level that may be undesirable for certain applications. For example, a work machine having hydraulic implements may require a predetermined engine speed to produce hydraulic pressures or flow sufficient to effectively manipulate the implements. A drop in engine speed during a downshift while simultaneously using the hydraulic implements could result in a drop in pressure and/or flow of the fluid supplied to the implements. The hydraulic pressure drop could cause a drop in implement force, while the drop in flow could cause a loss in implement speed, both possibly resulting in an overall less efficient operation of the work machine.
The disclosed transmission control system is directed to overcoming one or more of the problems set forth above.